Is it okay to leave my church because I don't agree with a building project?

There are several questions I would need to ask you before I could effectively answer your question. They are: How long have you spent praying about this building project? How many hours do you think the pastor and the board have spent praying about it? Do you not agree with it because of some moral issue, or are you not in favor of the project because you have a lack of faith in what God can accomplish? Lastly, and probably most importantly: Over the past few years, how many decisions of the church have you
fought or disagreed with?

Those questions are very important because it reveals the condition of your heart. If you have truly prayed about this project and the direction of the church, and still believe that it is wrong, you need to go to the pastor with your concerns and see how he responds. Just because you don't agree with a decision does not mean that you need to leave the church.

But, if in answering the above questions, you realize that you have a control issue or a problem submitting to the authority of the church, you need to let God deal with you on that. Whatever the case, do nothing without prayer and guidance from the Lord

"Humble yourselves therefore under the mighty hand of God, that he may exalt you in due time: Casting all your care upon him; for he careth for you." (I Peter 5:6-7)